An important role of the kidney is to maintain water and electrolyte balance. The function is in part regulated by the permeability properties of the renal tubules, and in the case of water transport, the renal collecting tubules under the influence of antidiuretic hormone (ADH). Alterations in the ability of ADH to regulate the renal tubule filtrate may lead to disease states, including hypertension. It has been suggested that calcium ion may play a major role in ADH-stimulation either as a part of the initial signal or in the transduction of this signal. Changes in cell morphology, especially apical plasma membrane, of the epithelial cells of the amphibian urinary bladder are frequently cited as initial evidence for such transformations following hormone stimulation. These morphological alterations appear to occur simultaneously to the appearance and alignment of microfilaments, intermediate filaments and microtubules. The structural and biochemical bases for this mobilization is poorly understood. The specific aim of the current research proposal is to evaluate the role of intracellular calcium in the membrane transformations and cytoskeletal mobilizations accompanying the action of anticuretic hormone using the amphibian urinary bladder epithelium as a model for the collecting tubules of the mammalian nephron. The principal hypothesis is to demonstrate that the action of ADH with its receptor is coupled to alterations in cell surface topography and cytoskeletal components by changes in intracellular calcium concentrations, and that calcium may regulate the permeability changes induced by ADH. The effects of ADH and agents that alter calcium movements (calcium ionophore and calcium antagonists) on the morphology and cytoskeletal components of epithelial cells will be examined using scanning and transmission electron microscopy coupled to immunofluorescent and immunocathodoluminescent cytoskeletal labelling techniques and freeze-etching techniques to monitor effects on intramembranous particle aggregates. In addition, alterations in calcium concentrations will be measured using electron energy loss and x- ray energy dispersive analysis of these cells. This study may lead to the development of a potential cellular-molecular model for the actions of ADH on transepithelial water flow.